The isoprenoids comprise the largest family of natural products with over 20,000 individual compounds described to date (Connolly, J. D. & Hill, R. A., Dictionary of Terpenoids (Chapman and Hall, London, 1991)). The isoprenoids play numerous functional roles in plants as hormones (gibberelins, abscisic acid), photosynthetic pigments (side chain of phytol carotenoids), electron carriers (side chain of plastoquinone), and structural components of membranes (phytosterols). Isoprenoids also serve in communication and defense, for example as attractants for pollinators and seed dispersers, and as competitive phytotoxins, antibiotics, and herbivore repellents and toxins (Harborne, J. B. in Ecological Chemistry and Biochemistry of Plant Terpenoids (Harborne, J. B., Tomas-Berbean, F. A, Eds.), pp. 399-426 (Clarendon Press, Oxford, 1991)).
Until recently, it was generally assumed that all isoprenoids were synthesized from acetyl-CoA via the classical mevalonate pathway (Spurgeon, S. L. & Porter, J. W., Eds., in Biosynthesis of Isoprenoid Compounds, Vol. 1, pp 1-46 (John Wiley, New York, 1983)). However, in 1993, Rohmer and co-workers (Rohmer, M. et al., Biochem. J. 295:517-524 (1993)) demonstrated that a non-mevalonate pathway, originating from pyruvate and glyceraldehyde-3-phosphate (GAP) (Rohmer, M. et al., J. Am. Chem. Soc. 118:2564-2566 (1996)), operated in several eubacteria, including E. coli. Evidence subsequently emerged that the plastid-derived isoprenoids of plants, including carotenoids and the prenyl side chains of chlorophyll and plastoquinone (Lichtenthaler, H. K. et al., FEBS Lett. 400:271-274 (1997)), as well as isoprene (Zeidler J. G. et al., Z. Naturforsch 52c:15-23 (1997)), monoterpenes (Eisenreich, W. et al., Tetrahedron Lett. 38:3889-3892 (1997)) and diterpenes (Eisenreich, W. et al., Proc. Natl. Acad. Sci. USA 93:6431-6436 (1996)); (Schwarz, M. K., PhD thesis, ETH, Zurich, Switzerland (1994)), are synthesized via the pyruvate/GAP route to isopentenyl diphosphate (IPP). This new pathway had been completely overlooked in the past.
The first dedicated reaction of this new enzymatic pathway to IPP is considered to involve a transketolase-type condensation involving pyruvate and GAP to form 1-deoxy-D-xylulose-5-phosphate (Rohmer, M. et al., J. Am. Chem. Soc. 118:2564-2566 (1996)); (Zeidler J. G. et al., Z. Naturforsch. 52c:15-23 (1997)); (Broers, S. T. J., PhD thesis, ETH, Zurich, Switzerland (1994)) (FIG. 1). A recent abstract has described the cloning of a gene encoding 1-deoxyxylulose-5-phosphate synthase from E. coli, but no sequence information, or other descriptive information, was reported (Lois, L. M. et al., Third Terpnet Meeting of the European Network on Plant Isoprenoids, Poitiers, France, May 29-30 (1997)).